Silicon in mechanical sensors

Silicon Cantilever for Micro/Nanoforce and Stiffness Calibration

The paper deals with cantilevers made from monocrystalline silicon by processes of microtechnology. The cantilevers are passive structures and have no transducers. The application as a material measure for the inspection of stylus forces is in the center of investigations. A simple method is the measurement of the deflection of the cantilever at the position of load by the force if the stiffness of the cantilever at this position is known. Measurements of force-deflection characteristics are described and discussed in context with the classical theory of elastic bending. The methods of determining the stiffness are discussed together with results. Finally, other methods based on tactile measurements along the cantilever are described and tested. The paper discusses comprehensively the properties of concrete silicon chips with cantilevers to underpin its applicability in industrial metrology. The progress consists of the estimation of the accuracy of the proposed method of stylus force measurement and the extraction of information from a tactile measured profile along the silicon cantilever. Furthermore, improvements are proposed for approaches to an ideal cantilever.

Compact Biomimetic Hair Sensors Based on Single Silicon Nanowires for Ultrafast and Highly-Sensitive Airflow Detection

Wearable sensors that can mimic functionalities of human bodies have attracted intense recent attention. However, research on wearable airflow sensors is still lagging behind. Herein, we report a biomimetic hair sensor based on a single ultralong silicon nanowire (SiNW-BHS) for airflow detection. In our device, the SiNW can provide both mechanical and electrical responses in airflow, which enables a simple and compact design. The SiNW-BHSs can detect airflow with a low detection limit (<0.15 m/s) and a record-high response speed (response time <40 ms). The compact design of the SiNW-BHSs also enables easy integration of an array of devices onto a flexible substrate to mimic human skin to provide comprehensive airflow information including wind speed, incident position, incident angle, and so forth. This work provides novel-designed BHSs for ultrafast and highly sensitive airflow detection, showing great potential for applications such as e-skins, wearable electronics, and robotics.

Ultraminiaturized Stretchable Strain Sensors Based on Single Silicon Nanowires for Imperceptible Electronic Skins

Miniaturized stretchable strain sensors are key components in E-skins for applications such as personalized health-monitoring, body motion perception, and human-machine interfaces. However, it remains a big challenge to fabricate miniaturized stretchable strain sensors with high imperceptibility. Here, we reported for the first time novel ultraminiaturized stretchable strain sensors based on single centimeter-long silicon nanowires (cm-SiNWs). With the diameter of the active materials even smaller than that of spider silks, these sensors are highly imperceptible. They exhibit a large strain sensing range (>45%) and a high durability (>10 000 cycles). Their optimum strain sensing ranges could be modulated by controlling the prestrains of the stretchable cm-SiNWs. On the basis of this capability, sensors with appropriate sensing ranges were chosen to respectively monitor large and subtle human motions including joint motion, swallow, and touch. The strategy of applying single cm-SiNWs in stretchable sensors would open new doors to fabricate ultraminiaturized stretchable devices.

Large area flexible pressure/strain sensors and arrays using nanomaterials and printing techniques

Sensors are becoming more demanding in all spheres of human activities for their advancement in terms of fabrication and cost. Several methods of fabrication and configurations exist which provide them myriad of applications. However, the advantage of fabrication for sensors lies with bulk fabrication and processing techniques. Exhaustive study for process advancement towards miniaturization from the advent of MEMS technology has been going on and progressing at high pace and has reached a highly advanced level wherein batch production and low cost alternatives provide a competitive performance. A look back to this advancement and thus understanding the route further is essential which is the core of this review in light of nanomaterials and printed technology based sensors. A subjective appraisal of these developments in sensor architecture from the advent of MEMS technology converging present date novel materials and process technologies through this article help us understand the path further.

Mechanically strengthened graphene-Cu composite with reduced thermal expansion towards interconnect applications

High-density integration technologies with copper (Cu) through-silicon via (TSV) have emerged as viable alternatives for achieving the requisite integration densities for the portable electronics and micro-electro-mechanical systems (MEMSs) package. However, significant thermo-mechanical stresses can be introduced in integrated structures during the manufacturing process due to mismatches of thermal expansion and the mechanical properties between Cu and silicon (Si). The high-density integration demands an interconnection material with a strong mechanical strength and small thermal expansion mismatch. In this study, a novel electroplating method is developed for the synthesis of a graphene-copper (G-Cu) composite with electrochemically exfoliated graphenes. The fabrication and evaluation of the G-Cu composite microstructures, including the microcantilevers and micromirrors supported by the composite, are reported. We evaluated not only the micromechanical properties of the G-Cu composite based on in-situ mechanical resonant frequency measurements using a laser Doppler vibrometer but also the coefficients of thermal expansion (CTE) of the composite based on curvature radius measurements at a temperature range of 20-200 °C. The Young's modulus and shear modulus of the composite are approximately 123 and 51 GPa, which are 1.25 times greater and 1.22 times greater, respectively, than those of pure Cu due to the reinforcement of graphene. The G-Cu composite exhibits a 23% lower CTE than Cu without sacrificing electrical conductivity. These results show that the mechanically strengthened G-Cu composite with reduced thermal expansion is an ideal and reliable interconnection material instead of Cu for complex integration structures.

Capacitive pressure sensing with suspended graphene-polymer heterostructure membranes

We describe the fabrication and characterisation of a capacitive pressure sensor formed by an ultra-thin graphene-polymer heterostructure membrane spanning a large array of micro-cavities each up to 30 μm in diameter with 100% yield. Sensors covering an area of just 1 mm² show reproducible pressure transduction under static and dynamic loading up to pressures of 250 kPa. The measured capacitance change in response to pressure is in good agreement with calculations. Further, we demonstrate high-sensitivity pressure sensors by applying a novel strained membrane transfer and optimising the sensor architecture. This method enables suspended structures with less than 50 nm of air dielectric gap, giving a pressure sensitivity of 123 aF Pa^-1 mm^-2 over a pressure range of 0 to 100 kPa.

Two-probe versus van der Pauw method in studying the piezoresistivity of single-wall carbon nanotube thin films

The use of the van der Pauw (VDP) method for characterizing and evaluating the piezoresistive behavior of carbon nanomaterial enabled piezoresistive sensors have not been systematically studied. By using single-wall carbon nanotube (SWCNT) thin films as a model system, herein we report a coupled electrical-mechanical experimental study in conjunction with a multiphysics finite element simulation as well as an analytic analysis to compare the two-probe and VDP testing configuration in evaluating the piezoresistive behavior of carbon nanomaterial enabled piezoresistive sensors. The key features regarding the sample aspect ratio dependent piezoresistive sensitivity or gauge factor were identified for the VDP testing configuration. It was found that the VDP test configuration offers consistently higher piezoresistive sensitivity than the two-probe testing method.

Ultra-thin graphene-polymer heterostructure membranes

The fabrication of arrays of ultra-thin conductive membranes remains a major challenge in realising large-scale micro/nano-electromechanical systems (MEMS/NEMS), since processing-stress and stiction issues limit the precision and yield in assembling suspended structures. We present the fabrication and mechanical characterisation of a suspended graphene-polymer heterostructure membrane that aims to tackle the prevailing challenge of constructing high yield membranes with minimal compromise to the mechanical properties of graphene. The fabrication method enables suspended membrane structures that can be multiplexed over wafer-scales with 100% yield. We apply a micro-blister inflation technique to measure the in-plane elastic modulus of pure graphene and of heterostructure membranes with a thickness of 18 nm to 235 nm, which ranges from the 2-dimensional (2d) modulus of bare graphene at 173 ± 55 N m^-1 to the bulk elastic modulus of the polymer (Parylene-C) as 3.6 ± 0.5 GPa as a function of film thickness. Different ratios of graphene to polymer thickness yield different deflection mechanisms and adhesion and delamination effects which are consistent with the transition from a membrane to a plate model. This system reveals the ability to precisely tune the mechanical properties of ultra-thin conductive membranes according to their applications.

Design and Analysis of a New Tuning Fork Structure for Resonant Pressure Sensor

This paper presents a micromachined resonant pressure sensor. The sensor is designed to optimize the sensitivity and reduce the cross-talk between the driving electrodes and sensing electrodes. The relationship between the sensitivity of the sensor and the main design parameters is analyzed both theoretically and numerically. The sensing and driving electrodes are optimized to get both high sensing capacitance and low cross-talk. This sensor is fabricated using a micromachining process based on a silicon-on-insulator (SOI) wafer. An open-loop measurement system and a closed-loop self-oscillation system is employed to measure the characteristics of the sensor. The experiment result shows that the sensor has a pressure sensitivity of about 29 Hz/kPa, a nonlinearity of 0.02%FS, a hysteresis error of 0.05%FS, and a repeatability error of 0.01%FS. The temperature coefficient is less than 2 Hz/°C in the range of -40 to 80 °C and the short-term stability of the sensor is better than 0.005%FS.

Plastic Deformation of Micromachined Silicon Diaphragms with a Sealed Cavity at High Temperatures

Single crystal silicon (SCS) diaphragms are widely used as pressure sensitive elements in micromachined pressure sensors. However, for harsh environments applications, pure silicon diaphragms are hardly used because of the deterioration of SCS in both electrical and mechanical properties. To survive at the elevated temperature, the silicon structures must work in combination with other advanced materials, such as silicon carbide (SiC) or silicon on insulator (SOI), for improved performance and reduced cost. Hence, in order to extend the operating temperatures of existing SCS microstructures, this work investigates the mechanical behavior of pressurized SCS diaphragms at high temperatures. A model was developed to predict the plastic deformation of SCS diaphragms and was verified by the experiments. The evolution of the deformation was obtained by studying the surface profiles at different anneal stages. The slow continuous deformation was considered as creep for the diaphragms with a radius of 2.5 mm at 600 °C. The occurrence of plastic deformation was successfully predicted by the model and was observed at the operating temperature of 800 °C and 900 °C, respectively.

Large-scale assembly of highly sensitive Si-based flexible strain sensors for human motion monitoring

Silicon is the dominant semiconductor in modern society, but the rigid nature of most Si structures hinders its applications in flexible electronics. In this work, Si-based flexible strain sensors are fabricated with Si fabric consisting of long Si nanowires. The as-obtained sensors demonstrate a large strain range of 50% and a gauge factor of up to 350, which are sufficient to detect human motions with superior performance over traditional sensors. The results reveal that the assembling strategy may potentially be applied to large-scale fabrication of highly sensitive, flexible strain sensors for emerging applications such as healthcare and sports monitoring. Moreover, the Si fabric would also enable broad applications of Si materials in other flexible and wearable devices such as flexible optoelectronics and displays.

Bio-Inspired Stretchable Absolute Pressure Sensor Network

A bio-inspired absolute pressure sensor network has been developed. Absolute pressure sensors, distributed on multiple silicon islands, are connected as a network by stretchable polyimide wires. This sensor network, made on a 4'' wafer, has 77 nodes and can be mounted on various curved surfaces to cover an area up to 0.64 m × 0.64 m, which is 100 times larger than its original size. Due to Micro Electro-Mechanical system (MEMS) surface micromachining technology, ultrathin sensing nodes can be realized with thicknesses of less than 100 µm. Additionally, good linearity and high sensitivity (~14 mV/V/bar) have been achieved. Since the MEMS sensor process has also been well integrated with a flexible polymer substrate process, the entire sensor network can be fabricated in a time-efficient and cost-effective manner. Moreover, an accurate pressure contour can be obtained from the sensor network. Therefore, this absolute pressure sensor network holds significant promise for smart vehicle applications, especially for unmanned aerial vehicles.

25th anniversary article: The evolution of electronic skin (e-skin): a brief history, design considerations, and recent progress

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

Effect of the molecular weight on deformation states of the polystyrene film by AFM single scanning

Nanobundles patterns can be formed on the surface of most thermoplastic polymers when the atomic force microscope (AFM)-based nanomechanical machining method is employed to scratch their surfaces. Such patterns are reviewed as three-dimensional sine-wave structures. In the present study, the single-line scratch test is used firstly to study different removal states of the polystyrene (PS) polymer with different molecular weights (MWs). Effects of the scratching direction and the scratching velocity on deformation of the PS film and the state of the removed materials are also investigated. Single-wear box test is then employed to study the possibility of forming bundle structures on PS films with different MWs. The experimental results show that the state between the tip and the sample plays a key role in the nano machining process. If the contact radius between the AFM tip and the polymer surface is larger than the chain end-to-end distance, it is designated as the "cutting" state that means the area of both side ridges is less than the area of the groove and materials are removed. If the contact radius is less than the chain end-to-end distance, it is designated as the "plowing" state that means the area of both side ridges is larger than the area of the groove and no materials are removed at all. For the perfect bundles formation on the PS film, the plowing state is ideal condition for the larger MW polymers because of the chains' entanglement.